1. Field of the Invention
The present invention relates to a method of reliably manufacturing hollow billets made of nonferrous metals--in particular, molten aluminum alloys having various compositions--by use of a vertical semi-continuous hot top casting method (to hereinafter be referred to simply as a hot top casting method), and an apparatus for manufacturing the same.
2. Description of the Related Art
The hot top casting method and the direct chill casting method are both known conventional methods used for forming billets by way of casting, for example, aluminum and alloys thereof.
A typical hot top casting method is described in Japanese Patent Publication No. 54-42847. According to the method described therein, a large quantity of molten metal is stored beside an upper refractory structure and is solidified by a lower water-cooled mold. This method permits the manufacture of high-quality billets which are free from internal defects. In this case, the billets manufactured are solid billets which are subsequently extruded.
When pipes are to be manufactured by way of mandrel extrusion, the extrusion billet should preferably be hollow in order to obtain a higher yield and for ease of manufacture. Consequently, there is now considerable demand for the development of a method by means of which billets can be manufactured in hollow form.
Attempts have been made to manufacture hollow billets by use of the hot top casting method. The hot top casting method is characterized in that a large quantity of molten metal is stored beside an upper refractory structure.
For this reason, contraction upon solidification of molten metal occurs in the hollow portion of the billet during the solidification process. A force for drawing a core into a billet is always generated during casting. As a result, if the core was drawn into the billet during casting, a large quantity of molten metal is poured on cooling water to often cause a steam explosion. The hot top casting method is not used in practice to manufacture hollow billets.
Attempts have also been made to manufacture hollow billets by use of the direct chill casting method. According to this method, a turbulence occurs in molten aluminum by a floating distributor and a spout of a movable part for adjusting a molten surface level. As a result, an oxide produced by the above turbulence inevitably enters the hollow billets, degrading the quality of the billets produced.
As a result of extensive studies carried out by the present inventors in relation to the problems experienced when using the above manufacturing methods, a "method of manufacturing hollow billets and an apparatus therefor" was developed and subsequently presented as Japanese Patent Application No. 62-107749 which constitutes the priority document of the present application.
According to this method, a core is positioned at the center of a mold and the distal end of the core is projected from the solidifying portion of the molten metal, thereby manufacturing hollow billets. However, when graphite is used to form the entire core of a lower part of the core, the surface of the graphite core becomes thermally worn and is corroded and degraded. The degraded graphite core surface has a large friction coefficient against the hollow portion to roughen the inner surface of the hollow portion. It is found that an accident such as leakage of the molten metal may eventually occur. It is also found that such a variation in molten surface level occurs particularly in the initial period of casting.